The present invention relates to methods and apparatus for treating swellable media in inkjet printing. In particular, it relates to methods and apparatus for pretreating media with fluid and/or heat immediately before printing to accelerate ink penetration and reduce coalescence.
Inkjet print media comprising a swellable ink-receiving layer (e.g., gelatin and other hydrogels such as polyvinylpyrrolidone and copolymers including polyvinyl alcohol or polyethylene oxide) are commonly used for inkjet photo-imaging. These media (typically coated papers) tend to provide better lightfastness and durability than porous media (such as plain or coated paper), and sometimes improve image quality, as well. However, swellable ink-receiving layers are frequently subject to a problem of slow ink uptake. The time it takes for an ink-receiving layer to swell and absorb ink from a surface is often longer than the time needed for a porous medium to absorb ink. At high print speeds, ink may puddle or xe2x80x9ccoalescexe2x80x9d before it is fully absorbed, limiting achievable image quality.
It is an object of the present invention to provide an inexpensive method and apparatus to reduce coalescence and improve image quality.
In one aspect, the invention comprises a method of improving print quality for a printer that prints to a swellable medium. The method comprises applying treatment fluid to the swellable medium, no more than one minute before the ink is applied to the medium. This may be accomplished, for example, by the use of an in-line apparatus that applies the treatment fluid to the medium as it passes through the printer, before it reaches the print head, for example by a roller, a wiper, a sprayer or an inkjet printhead. The time between application of the treatment fluid and the ink to the medium may be, for example, no greater than fifteen seconds, five seconds, or one second. The medium may be heated while or immediately after the treatment fluid is applied. The treatment fluid may be selected to increase the hydrophilicity of the swellable medium. For example, it may comprise a polar solvent (e.g., water) and a wetting agent or a surfactant. It may further contain drying agents. In some embodiments, the treatment fluid comprises about 5-30% alcohols and/or diols (e.g., about 8-20% 1,2-hexanediol and about 2-5% 1-butanol), about 1-4% surfactant (e.g., about 1-4% secondary alcohol ethoxylate), and about 71-89% water.
In another aspect, the invention comprises a method of improving image quality by applying a treatment fluid to a print medium, where the treatment fluid is selected to cause rapid precipitation of a colorant from ink used to print to the print medium. The colorant may be, for example, a pigment or a dye. The treatment fluid and the ink may be selected so that the colorant forms an insoluble salt with the treatment fluid, thereby precipitating the colorant.
In a further aspect, the invention comprises a printer for applying a treatment fluid to print media. The printer comprises a pretreatment applicator than applies the treatment fluid, and an ink jet that applies ink no more than about one minute after the treatment fluid is applied. The printer may also comprise a feeder that feeds the print media over the pretreatment applicator before it is brought into communication with the inkjet. The pretreatment applicator may comprise, for example, a roller (e.g., a microporous roller) and a fluid reservoir, where the passage of the print media over the applicator causes the roller to apply fluid from the reservoir to the print media. Optionally, the printer may also comprise a heater that heats the print media adjacent to the pretreatment applicator.